首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   5942篇
  免费   1200篇
  国内免费   1807篇
测绘学   153篇
大气科学   427篇
地球物理   1718篇
地质学   4947篇
海洋学   640篇
天文学   81篇
综合类   331篇
自然地理   652篇
  2024年   59篇
  2023年   195篇
  2022年   285篇
  2021年   298篇
  2020年   291篇
  2019年   330篇
  2018年   292篇
  2017年   286篇
  2016年   280篇
  2015年   310篇
  2014年   376篇
  2013年   362篇
  2012年   411篇
  2011年   405篇
  2010年   337篇
  2009年   383篇
  2008年   374篇
  2007年   402篇
  2006年   392篇
  2005年   307篇
  2004年   317篇
  2003年   265篇
  2002年   211篇
  2001年   193篇
  2000年   204篇
  1999年   154篇
  1998年   189篇
  1997年   202篇
  1996年   156篇
  1995年   131篇
  1994年   114篇
  1993年   85篇
  1992年   94篇
  1991年   54篇
  1990年   59篇
  1989年   49篇
  1988年   35篇
  1987年   12篇
  1986年   24篇
  1985年   7篇
  1984年   1篇
  1983年   3篇
  1982年   2篇
  1980年   4篇
  1978年   2篇
  1977年   2篇
  1976年   1篇
  1954年   4篇
排序方式: 共有8949条查询结果,搜索用时 15 毫秒
41.
Ti separation was achieved by ion-exchange chromatography using Bio-Rad AG 1-X8 anion-exchange and DGA resins. For high-Fe/Ti and high-Mg/Ti igneous samples, a three-column procedure was required, whereas a two-column procedure was used for low-Fe/Ti and low-Mg/Ti igneous samples. The Ti isotopes were analysed by MC-ICP-MS, and instrumental mass bias was corrected using a 47Ti-49Ti double-spike technique. The 47Ti-49Ti double-spike and SRM 3162a were calibrated using SRM 979-Cr, certificated value 53Cr/52Crtrue = 0.11339. Isobaric interference was evaluated by analysing Alfa-Ti doped with Na, Mg, Ca, and Mo, and results indicate that high concentrations of Na and Mg have no significant effect on Ti isotope analyses; however, Ca and Mo interferences lead to erroneous δ49/47Ti values when Ca/Ti and Mo/Ti ratios exceed 0.01 and 0.1, respectively. Titanium isotopic compositions were determined for 12 igneous reference materials, BCR-2, BHVO-2, GBW07105, AGV-1, AGV-2, W-2, GBW07123, GBW07126, GBW07127, GBW07101, JP-1, and DTS-2b. Samples yield δ49/47Ti (‰) of ?0.035 ± 0.022, ?0.038 ± 0.031, 0.031 ± 0.022, 0.059 ± 0.038, 0.044 ± 0.037, 0.000 ± 0.015, 0.154 ± 0.044, ?0.044 ± 0.018, 0.010 ± 0.022, 0.064 ± 0.043, 0.169 ± 0.034, and ?0.047 ± 0.025 (relative to OL-Ti, ±2SD), respectively; of which isotopic compositions of DTS-2b, JP-1, GBW07101, GBW07105, GBW07123, GBW07126, and GBW07127 are reported for the first time. Standard Alfa-Ti was analysed repeatedly over a ten-month period, indicating a reproducibility of ±0.047 (2SD) for δ49/47Ti, similar to the precisions obtained for geochemical reference materials.  相似文献   
42.
Through a systematic observation of water level and temperature, and a comprehensive analysis of the data on major/trace elements, nitrite, hydrogen-oxygen isotopes, the conclusion has been drawn that there are two relatively independent groundwater systems (cool water and hot water), and the geochemical indicators of hot/cool waters are described. The cool water system is relatively enriched in Ca2 , Mg2 and HCO3-. Its TDS is relatively low, about 1400–1800 mg/L. The hot water system is relatively enriched in K , Na , Cl- and SO42-. Its TDS is relatively high, about 2200–2300 mg/L. The cool water system is enriched in Ba, Ga, Cd, and the hot water system is enriched in B, Ti, Cr, Ni, Cu, Mo, Rb, and Cs, relatively. Especially, the contents of Rb and Cs in the hot water system are more than five times as high as those in the cool water system. The NO3- contents of cool water discharged from the gold mine are relatively high, and those of hot water are extremely low. The δD and δ18O values follow an increasing order of surface water>mine cool water>mine hot water. The cool water comes mainly from the lateral supply of phreatic water, while the hot water comes mainly from the vertical supply of deeply circulating structure-fracture water. The ratio of cool water over hot water was estimated to be about 1:1 by a water quality model..  相似文献   
43.
根据笔者在柴达木盆地侏罗系生油层和原生油藏的研究成果,指出我国应重视侏罗系油源的战略性侦察以及古生代及其以前海相油源战略性的研究。   相似文献   
44.
A study of faults and their control of deep gas accumulations has been made on the basis of dividing fault systems in the Xujiaweizi area. The study indicates two sets of fault systems are developed vertically in the Xujiaweizi area, including a lower fault system and an upper fault system. Formed in the period of the Huoshiling Formation to Yingcheng Formation, the lower fault system consists of five fault systems including Xuxi strike-slip extensional fault system, NE-trending extensional fault system, near-EW-trending regulating fault system, Xuzhong strike-slip fault system and Xudong strike-slip fault system. Formed in the period of Qingshankou Formation to Yaojia Formation, the upper fault system was affected mainly by the boundary conditions of the lower fault system, and thus plenty of multi-directionally distributed dense fault zones were formed in the T2 reflection horizon. The Xuxi fault controlled the formation and distribution of Shahezi coal-measure source rocks, and Xuzhong and Xudong faults controlled the formation and distribution of volcanic reservoirs of Y1 Member and Y3 Member, respectively. In the forming period of the upper fault system, the Xuzhong fault was of successive strong activities and directly connected gas source rock reservoirs and volcanic reservoirs, so it is a strongly-charged direct gas source fault. The volcanic reservoir development zones of good physical properties that may be found near the Xuzhong fault are the favorable target zones for the next exploration of deep gas accumulations in Xujiaweizi area.  相似文献   
45.
南水北调中线水源地河水地球化学特征与流域侵蚀   总被引:1,自引:0,他引:1  
丹江口水库及其上游流域是南水北调中线工程的水源地,本文讨论了水源地河流水化学与锶同位素(87Sr/86Sr)组成变化特征,目的在于了解水源地流域河流地表水溶质的物质来源以及岩石风化侵蚀过程和人为活动的影响。流域内河流水化学组成以Ca2+、HCO3-为主,Mg2+和SO42-次之,反映了碳酸盐岩风化溶解起控制作用的典型特征。水化学分析表明水源地河水受到工农业活动等人为因素的影响;河流87Sr/86Sr同位素地球化学研究表明,流域岩石风化输入至少存在三个不同端员(硅酸岩、石灰岩和白云岩)之间的混合。水源地流域内硅酸岩和碳酸岩的风化侵蚀速率分别为38.6和4.4 t/km2.a,总岩石风化侵蚀速率高于全球河流平均值。  相似文献   
46.
A Metallogenic Model of Gold Deposits of the Jiaodong Granite-Greenstone Belt   总被引:34,自引:0,他引:34  
An analysis of trace elements and isotopic geochemistry suggest that the ore-forming materials of gold deposits in the Jiaodong granite-greenstone belt have multiple sources, especially the mantle source. Seismic wave, magnetic and gravity fields show that the crust-mantle structure and its coupling mechanism are the fundamental dynamic causes for the exchange and accumulation of materials and energy in the metallogenic system. Considering the evolution history of the structural setting, the tectono-metallogenic dynamics model of the area can be summarized as follows: (1) occurrence of the greenstone belt during the Archean-Proterozoic-the embryonic form of Au-source system; (2) stable tectonic setting in the Paleozoic-an intermittence in gold mineralization; (3) intensive activation and reformation of the greenstone belt in the Mesozoic-tectono-mineralization and tectono-diagensis; (4) posthumous structural activity in the Cenozoic-destruction of orebodies in the later stage. In the middle and late Ind  相似文献   
47.
48.
晶体及矿物颗粒大小对岩土材料力学性质的影响   总被引:1,自引:0,他引:1  
许尚杰  尹小涛  党发宁 《岩土力学》2009,30(9):2581-2587
岩石常见较大的晶体或者矿物颗粒,混凝土中是骨料,通过团簇模拟大颗粒的力学行为、团簇可以破裂。根据设计的相同数量、相同位置、不同半径的大颗粒数值单轴压缩试验,在虚拟试验条件下,考察了颗粒大小对材料力学响应的影响。通过分析颗粒材料的破裂形态、裂纹扩展过程、应力-应变曲线和破裂能量演化规律发现:大颗粒具有明显的增强特性,有阻止裂纹扩展的作用,破裂多绕大颗粒发展;增强幅度随颗粒半径的增加呈单增趋势,半径较小时,增强效果不明显。  相似文献   
49.
云南会泽超大型铅锌矿床成因研究中的几个问题   总被引:6,自引:0,他引:6  
位于川-滇-黔铅锌多金属成矿域中南部云南会泽超大型铅锌矿床可能是一种新的铅锌矿床类型,该类铅锌矿床明显特征是规模大、品位富、伴生有用元素多,暗示其成矿环境较为特殊。从成矿时代、成矿物质来源、成矿流体来源与演化,以及峨眉山玄武岩与成矿的关系等方面分析了会泽超大型铅锌矿床的研究进展及国内外研究现状,认为矿床成矿时代与西南大面积峨眉山玄武岩成矿时代相近,成矿物质和成矿流体具有“多源性”,成矿流体存在均一化过程,区域大规模流体运移在该区铅锌成矿过程中具有重要意义,峨眉山玄武岩岩浆活动与铅锌成矿具有密切的成因联系,矿床可能为“均一化成矿流体贯入成矿”的产物。  相似文献   
50.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号